“Mars can’t protect itself from the solar wind because it no longer has a shield, the planet’s global magnetic field is dead,” says the mission’s principal investigator Bruce Jakosky of the University of Colorado, Boulder.

“A better understanding of the upper atmosphere and the loss of volatile compounds like carbon dioxide, nitrogen dioxide, and water to space is required to plug a major hole in our understanding of Mars,” adds Jakosky.

Clues on the Martian surface, such as features resembling dry riverbeds and minerals that only form in the presence of liquid water, suggest that Mars once had a denser atmosphere, which supported the presence of liquid water on the surface.

Mars lost its global magnetic field in its youth billions of years ago. Once its planet-wide magnetic field disappeared, Mars’ atmosphere was exposed to the solar wind and most of it could have been gradually stripped away.

While the sun is the primary suspect in the loss of atmosphere, Mars also has more than 20 ancient craters larger than 600 miles across, scars from giant impacts by asteroids the size of small moons. This bombardment could have blasted large amounts of the Martian atmosphere into space.

However, huge Martian volcanoes that erupted after the impacts, like Olympus Mons, could have replenished the Martian atmosphere by venting massive amounts of gas from the planet’s interior.

It’s possible that the hijacked Martian air was an organized crime, with both impacts and the solar wind contributing. Without the protection of its magnetic shield, any replacement Martian atmosphere that may have issued from volcanic eruptions eventually would also have been stripped away by the solar wind.

As the Martian atmosphere thinned, the planet got drier as well, because water vapor in the atmosphere was also lost to space, and because any remaining water froze out as the temperatures dropped when the atmosphere disappeared.

MAVEN can discover how much water has been lost to space by measuring hydrogen isotope ratios.

MAVEN is scheduled for launch between November 18 and December 7, 2013. If it is launched November 18, it will arrive at Mars on September 16, 2014 for its year-long mission.

The project is part of NASA’s Mars Scout program, funded by NASA Headquarters in Washington, DC. The University of Colorado will coordinate the science team and science operations.